176 



ANIMAL BIOLOGY 



composed. The wastes are forced out through the larger canals 

 referred to above. Like the bath sponge, all other Porifera are 

 stationary in their mature form. 



B. Ccelenterata 



131. Hydra. A study of a fresh water ccelenterate known as 

 hydra will give one a fair idea of the structure and adaptations of 

 this group of animals. Hydra is a small animal found in fresh water 

 attached to water plants, and sometimes to surfaces of stones or 



tentacle 

 nettling cells 



young sperm-cells 

 digestive cavity 

 mature egg 



embryo.,''" 

 hydraa \ 



"~ base of hydra 

 FIG. 124. Longitudinal section of a hydra. (Hegner.) 



other objects on the bottom. At the upper end of the tiny cylin- 

 drical column are threadlike bodies known as tentacles (Fig. 125, 1}. 

 If the animal is touched with a needle or pencil, it contracts its 

 body and tentacles so much that it can scarcely be seen. But in a 

 short time it expands again. 



If the hydra happens to be hungry and some small form of animal 



_young 

 "tentacle 



